I used this tutorial myself on my venerable dreadnoughts, and i feel its by far the best and easiest method to make these arms.

Kits required:- One Dreadnought or Venerable Dreadnought; either one will do.- One Aegis Defence Line

Tools required:- cutters or saw; your preference- hobby knife- glue!

Time: 5-10 minutes

1) Trim out the following (circled in red) parts and clean off flashing (mold lines) and etc:- Aegis bits

2) Assemble the assault cannon arm from the Dreadnought kit as normal.

3) Cut off the assault cannon at its base, leaving the arm housing flat as shown by the red line here:

4) Time to cut down the autocannons. Follow the red lines here to see where to cut (though in the end it's your preference for length). Make sure the butts are flat so they sit on the dreadnought arm nicely; same goes for one autocannon's top and the other's bottom, so they fit together as well.

5) Slap it all together like thus (make sure the bottom of one fits the top of the other!):

this conversion uses only one half of the Aegis Defense Line's Autocannons, so each kit will actually net you 2 Arms worth of gun barrels.